俱舍論疏 22.201

22.201

論「隨謂繫心至念恒隨逐」，第二釋隨也。就隨之中，有隨入者、有隨出者，此明入也。

On the Śāstra's text, "Following means tying the mind ... mindfulness constantly follows along," this secondly explains following. Within following, there is following the in-breath and following the out-breath; this clarifies following the in-breath.